No. Skin doesn't know what time of day it is. Day cream may contain SPF which is an unnecessary application of chemicals at night, but it won't do any harm. The main benefit of having a separate night cream is that it tends to be thicker, giving skin a super-dose of hydration without you having to be bothered with the shine it creates. And that can be handy, too, in creating a barrier against moisture loss if you leave the central heating on at night.